1. Field of the Invention
The present invention relates to a temperature-compensated quartz crystal oscillator, and more particularly to a temperature-compensated crystal oscillator (TCXO) which employs a component layout that is designed for a highly reduced size.
2. Description of the Related Art
Quartz crystal units are incorporated as a time or frequency reference source in mobile communication devices such as cellular phones or the like, and temperature-compensated crystal oscillators are adopted to compensate for temperature-depending changes in the resonant frequency of such crystal units.
In recent years, there has been a growing demand for smaller temperature-compensated crystal oscillators in view of a quick tendency toward smaller mobile communication devices such as cellular phones or the like. Temperature-compensated crystal oscillators generally comprise a substrate, and a crystal unit and an oscillating circuit which are disposed on one surface of the substrate. A temperature-compensated crystal oscillator having a size of approximately 5 mm×7 mm has a mounting surface, i.e., the surface area of a wiring board for mounting crystal oscillator components thereon, large enough to place circuit components of the oscillating circuit and the crystal unit on one substrate even if they are arranged in a plane.
FIG. 1 shows by way of example a temperature-compensated crystal oscillator of the direct compensation type. The temperature-compensated crystal oscillator has IC (Integrated Circuit) 1 and crystal unit 6 connected to IC 1. IC 1 comprises an integrated assembly of two cascaded transistors, some resistors, and capacitors (not shown) which cooperate with crystal unit 6 in making up a Colpitts crystal oscillating circuit. IC 1 has power supply terminal Vcc for supplying electric power to various circuit components inside IC 1, oscillation input terminal X connected to one terminal of crystal unit 6, and output terminal Out for supplying an oscillation output to an external circuit. Crystal unit 6 may comprise an AT-cut quartz crystal unit, for example.
The oscillator also has two frequency-adjusting capacitors 7, 8, temperature-compensating circuit 9, variable capacitance diode 10, resistor 11 for applying a control voltage to the cathode of variable capacitance diode 10, and voltage-dividing resistor 12 connected parallel to variable capacitance diode 10. Capacitors 7, 8 are connected parallel to each other and have ends connected to a junction that is connected to the other terminal of crystal unit 6. The other ends of capacitors 7, 8 are connected to a terminal of temperature-compensating circuit 9, whose other terminal is connected to the cathode of variable capacitance diode 10. Variable capacitance diode 10 has an anode connected to ground.
Temperature-compensating circuit 9 comprises capacitor 13, high-temperature-compensating thermistor 14 connected parallel to capacitor 13, and a series-connected circuit of low-temperature-compensating thermistor 15 and capacitor 16 which are connected parallel to capacitor 13.
The oscillator has four electrodes, for example, for connection to external circuits. The four electrodes include ground electrode 2, power supply electrode 3 which is supplied with a power supply voltage, output electrode 4 where the oscillation output appears, and control electrode 5 to which control voltage Vc is applied for finely adjusting the oscillation frequency of the oscillator. Ground electrode 2 is connected to ground terminal E of IC 1 and the anode of variable capacitance diode 10. Power supply electrode 3 is connected to power supply terminal Vcc of IC 1. Output electrode 4 is connected to output terminal Out of IC 1. Control electrode 5 applies control voltage Vc through resistor 11 to the cathode of variable capacitance diode 10 for finely adjusting the oscillation frequency.
The temperature-compensated crystal oscillator shown in FIG. 1 is referred to as a temperature-compensated crystal oscillator of the direct compensation type because temperature-compensating circuit 9 is directly connected to the crystal unit. Other circuit arrangements of the temperature-compensated crystal oscillator of the direct compensation type are disclosed in Japanese laid-open utility model publication No. 61-81208 (JP. 61-81208, U) and Japanese laid-open patent publication No. 06-85538 (JP, 6-85538, A). There is also known a temperature-compensated crystal oscillator of the indirect compensation type wherein a variable capacitance diode is connected in series to a crystal unit, an output voltage of a temperature sensor is applied to a resistor circuit network or the like to generate a control voltage that changes depending on the temperature according to a cubic curve, and the generated control voltage is applied to the variable capacitance diode for temperature compensation.
The temperature-compensated crystal oscillator shown in FIG. 1 needs ten circuit components in addition to crystal unit 6. If the size of the mounting surface of the crystal oscillator Is reduced to about one-third, for example, of the present size of 5 mm×7 mm, then the surface area of the substrate is no longer large enough to place the crystal unit and the circuit components of the oscillating circuit thereon in a plane.